Compact embedding of a confining-potential energy space
= Compact embedding of a confining-potential energy space
The embedding $\Sigma_V\hookrightarrow L^2(\mathbb R^d)$ is compact. The <Rellich-Kondrachov compactness theorem> gives compactness on each fixed ball, while
$$
\int_{|x|>R}|u|^2
\leq\left(\inf_{|x|>R}V(x)\right)^{-1}
\int_{\mathbb R^d}V|u|^2
$$
makes the tails uniformly small.